1. Field of the Invention
This invention relates to numerical control systems for machines having movable members controlled by servosystems and more particularly to a resolver position measuring device in such a system for indicating the position of a movable member.
2. Description of the Prior Art
Numerical control systems control machines, such as milling and boring machines, lathes and the like wherein the machines have movable members that are moved by servosystems under the control of the numerical control system. The numerical control system typically commands movement of a member of the machine, measures the position of the member and then revises its command of the member in closed loop fashion. Often, numerical control systems control multiple axes of a machine in time coordinated fashion. The position of the movable member is typically measured using a resolver wherein quadrature excitation signals are applied to the stators of the resolver, the rotor of the resolver is operably connected to the movable member, and the waveform out of the rotatable rotor of the resolver is analyzed.
Such a resolver position measuring device for a numerical control system must be relatively inexpensive and capable of making numerous rapid, accurate, and reliable measurements while operating in a relatively hostile environment. For example, a typical numerical control system may require greater than 150 resolver position measurements per second on a resolver whose rotor may be revolving at a velocity of greater than 40,000 degrees per second. A high measurement rate is desirable because it is desirable to make a measurement less than every one-half revolution of the rotor when operating at maximum velocity of the movable member such that the direction of motion and absolute position can be kept track of by the electronics. Accurate measurements are desirable to provide precision machine movement and closed loop feedback control. Typically, the numerical control apparatus and the resolver position measuring device must operate with proper speed and accuracy in a machine-shoptype environment in which the ambient temperature can vary in an unpredictable manner.
U.S. Pat. No. 3,634,838 issued to Granqvist shows a resolver position measuring device that utilizes two counters and a resolver having three stator windings. The first counter generates a reference 400 Hz square wave which is filtered and then phase shifted to provide first and second sinusoidal stator winding excitations. The third stator winding detects the phase of at least one of the applied signals; and a phase detector compares the phase of the third stator winding to the phase of the second counter and activates circuitry to increase or decrease pulses to the first counter to maintain the third stator winding in phase with the second counter. A phase responsive device detects the zero crossing of the induced rotor waveform and utilizes it to transfer the count of the second counter to a storage device. As a practical matter, however, the implementation of such a closed loop phase compensating scheme may be relatively complex and expensive.
Another known resolver position measuring device applies quandrature square waves to the resolver's two stator windings. The waveform induced in the rotor is then filtered to pass the fundamental frequency and eliminate the higher order harmonics. A phase comparison loop compares the phase of this fundamental frequency to the phases of the square wave stator excitation and provides a count indicative of the phase difference. As a practical matter, however, an accurate temperature-compensated filter for such a resolver position measuring device is rather expensive.